1. Field of the Invention
The present invention relates generally to strain gauges and more particularly relates to strain gauges for medical applications.
2. Description of the Prior Art
It is desirable with a number of medical devices to measure mechanical motion. This is particularly true with regard to monitoring patients susceptible to apnea, or cessation of respiration. However, a number of other applications are readily apparent, such as monitoring cardiac activity, joint angles, joint motion, muscle or penile tumescence, etc.
Most frequently, direct monitoring of the mechanical motion is sufficiently difficult that the desired motion is monitored using an indirect measurement technique. Perhaps the most common is found in cardiac pacing wherein it is common to sense electrical activity of the myocardium and associate that with the presence or absence of the actual mechanical activity.
Such indirect monitoring is particularly prevalent in the treatment of respiratory diseases including apnea. Acoustic sensors for monitoring respiration are seen in U.S. Pat. No. 4,602,644 issued to DeBenedetto et al., and U.S. Pat. No. 4,595,016 issued to Fertig et al. Optoelectric flow sensors are described by Dietz in U.S. Pat. Nos. 4,878,502 and 4,745,925.
Other indirect monitoring techniques include gas monitoring in U.S. Pat. No. 4,366,821 issued to Wittmaier et al., and moisture sensing using a sodium chloride crystal as found in U.S. Pat. No. 4,326,404 issued to Mehta. U.S. Pat. No. 4,306,867 issued to Krasner shows the use of a pressure sensor. An impedance plethysmograph is employed in U.S. Pat. No. 4,289,142 issued to Kearns. The use of thermoresistive sensors is suggested in U.S. Pat. No. 3,903,876 issued to Harris; U.S. Pat. No. 3,884,219 issued to Richardson et al; and U.S. Pat. No. 3,999,537 issued to Noiles.
The preferred method of measuring mechanical activity directly is through the use of a strain gauge. The term strain gauge is typically used in the art to refer to such a device even though they tend to measure mechanical displacement rather than strain. Perhaps the term is more appropriately applied when the device monitors a mechanical spring within its elastic limits, wherein the measurement of distension or compression can readily be correlated to strain on the components of the mechanical spring.
Many strain gauges utilize the mechanical changes to vary the capacitive or inductive impedance of an alternating current path. This technique tends to provide a sensor element which is easily constructed. However, the electronic circuitry associated with such systems tends to be very complex. To simplify the electronics, strain gauges which vary direct current resistance with mechanical motion have been developed. U.S. Pat. No. 4,971,065 issued to Pearce shows such a system. However, these devices tend to have a rather poor signal-to-noise ratio because of the high resistivity of the overall sensor generated by the relatively long serpentine low conductivity path needed to generate sufficient resistance change to monitor the desired motion.